It is known to use free radical generating agents for modifying a product, such as a polymer composition via a radical reaction.
Free radical agents are used e.g. to initiate (a) crosslinking in a polymer, i.a. primarily formation of interpolymer crosslinks (bridges) by radical reaction, (b) grafting in a polymer, i.e. introduction of compounds to a polymer chain (to backbone and/or side chains) by radical reaction, and (c) visbreaking in a polymer, i.e. modification of melt flow rate (MFR) of a polymer by radical reaction. These polymer modifications are well known in the art.
When added to a polymer composition, free radical generating agents act by generating radicals, typically by decomposing to radicals, under conditions which enable the radical formation. The decomposed radicals initiate further radical reactions within a polymer composition. The resulting decomposition products of the free radical generating agent are typically a result of several reactions of the decomposition products of the initial radical forming reaction. Said resulting decomposition products typically remain in the modified polymer and may include detrimental, undesired decomposition products.
Peroxides are very common free radical generating agents used i.a. in the polymer industry for said polymer modifications. The resulting decomposition products of peroxides may include volatile by-products. For example, dicumylperoxide, which is commonly used peroxide in polymer field, decomposes i.a. to methane, acetophenone and cumylalcohol during the radical formation step, e.g. during a crosslinking step. The formed gaseous methane (CH4) is flammable, explosive and volatile and thus a risk in a working environment.
In wire and cable applications, a typical cable comprises at least one conductor surrounded by one or more layers of polymeric materials. In some power cables, including medium voltage (MV), high voltage (HV) and extra high voltage (EHV) cables, said conductor is surrounded by several layers including an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. One or more of said layers are then typically crosslinked to improve i.a. deformation resistance at elevated temperatures, as well as mechanical strength and/or chemical resistance, of the layer(s) of the cable. The free radical generating agent, such as peroxide, is typically incorporated into the layer material prior to the extrusion of the layer(s) on a conductor. After formation of the layered cable, the cable is then subjected to a crosslinking step to initiate the radical formation and thereby crosslinking reaction.
The decomposition products of the free radical forming agent remain mostly captured within the cable layer after crosslinking. This causes problems in view of the cable manufacturing process as well as in view of the quality of the final cable.
Accordingly, after crosslinking the cable must be cooled with great care to prevent the gaseous volatile decomposition products like methane forming voids within the polymer layer. These voids have typically an average diameter of between 10 to 100 μm. Partial discharges can take place in such voids within a cable that is subjected to an electrical field and thereby reduce the electrical strength of the cable.
The MV, HV and EHV power cables must have high layer quality in terms of safety during installation and in end use thereof. In service, volatile decomposition products in a cable resulting from a crosslinking step can create a gas pressure and thus cause defects in the shielding and in the joints. E.g. when a cable is equipped with a metal barrier, then the gaseous products can exert a pressure, especially on the joints and terminations, whereby a system failure may occur.
For the above reasons the volatile decomposition products, such as methane e.g. where dicumylperoxide is used, are conventionally reduced to a minimum or removed after crosslinking and cooling step. Such a removal step is generally known as a degassing step.
The degassing step is time and energy consuming and is thus a costly operation in a cable manufacturing process. Degassing requires large heated chambers which must be well ventilated to avoid the build-up of e.g. flammable methane and ethane. The cable, typically wound to cable drums, is normally degassed at elevated temperature in the range of 50-80° C., e.g. 60-70° C., for lengthy time periods. At these temperatures however, thermal expansion and softening of the insulation can occur and lead to undue deformation of the formed cable layers resulting directly in failures of the cable. The degassing of MV, HV and EHV cables with high cable weight needs thus often to be carried out at decreased temperatures.
Accordingly, there is a need to find new solutions to overcome the prior art problems.